Modern materials present a number of formidable challenges to the fabricators of optics and other precision components. There is a general need to handle high fluences and to produce lenses and mirrors that are highly aspheric. Any form of contact figuring or polishing introduces a degree of distortion into the workpiece. Present finishing technologies involve the creation of significant levels of surface and subsurface damage and are difficult to apply to non-spherical shapes. In spite of 5000 years of development, final figuring and polishing is still largely a black art. The scientific principles controlling current systems are very complicated and involve surface and solution chemistry as well as mechanical abrasion. There are continuing requirements to have cost effective apparatus and methods of super finishing for a number of micro and precision components along with the more traditional laboratory demand for extraordinary one-of-a-kind optics. Many quality optics are currently available only in limited numbers; but, are restricted in their use because of the high cost. In many applications, mechanically inferior or more costly materials are used because the material of first choice (in terms of performance) is too difficult to fabricate.
In demanding applications, the success of any manufactured precision component depends on the surface and near-surface quality. Damage, present in crystalline or amorphous substrates as cracks, voids or plastically deformed material can lower the damage threshold for high fluence use or increase the chemical activity of the surface (corrosion). A shaping and finishing process that involves surface contact will introduce damage to some degree. In addition, for silica glass optics, high quality final finishing is accomplished through complex solution phase chemistry that tends to deposit contaminants in a gel-like layer on the surface of the component. While an extremely smooth part can be produced, the top layer often hides an extensive damage layer below. An optimum approach to final surface generation would involve a non-contact system that removes material chemically without residual contamination. A number of attempts at non-contact material removal have been reported.
Plasma etching at reduced pressure is extensively used in the semiconductor industry for processing of a wide variety of materials including semiconductors, metals and glasses (1). The references cited herein are listed as end notes and are all incorporated by reference. Removal mechanisms and removal rates have been studied extensively and are reasonably well understood. With pressures in the 10 to 20 millitorr range, ion and electron densities are on the order of 109 to 1010 cm−2. These reactive ions are believed responsible for the majority of material removal. Consequently, the technique is known as reactive ion etch (RIE). With electron energies in the 3 to 30 eV range, material removal tends to be largely chemical in nature. Below 50 eV physical sputtering is negligible and subsurface damage is non-existent. In the absence of sputtering, reaction products must be volatile or the process is self terminating after the formation of the first product layer.
Considerable effort has been put into developing plasmas with uniform etch rates over the entire discharge making RIE unsuitable for the production of figured precision components. However, the greatest practical drawback to the technique for cost-effective fabrication and finishing would be the low rate of material removal and the requirement of a vacuum. Translating either the source or workpiece with precision in a complicated pattern inside a vacuum chamber is challenging, especially in the case of large optics. In-situ metrology can also be awkward. Fortunately, the chemistry that occurs in RIE systems is similar to that of higher pressure systems. The main difference is the population shift between atom and ion species in the tail of the high pressure discharge. There, atoms produced by ion-electron recombination reactions predominate. As a result, reactive gas mixtures that are used in RIE will also work for atmospheric systems.
A modified RIE for polishing at reduced pressure has been built using a capacitively coupled discharge (2-5). Named “Plasma Assisted Chemical Machining (PACE), the system has been successful in shaping and polishing fused silica. In the first application of PACE, Bollinger et. al. (2) noted that a 1″ discharge controlled material removal to better than 1% with removal rates from 0 to 10 um/min. The footprint and the rates could be varied during the process by changing plasma parameters such as power and reactive gas flow. In subsequent analysis, the parts showed no evidence of subsurface damage, no surface contamination and no distortion at the edge of the optic (roll-off). Material removal could be predicted from a linear superposition of the static footprint. Control algorithms were developed to permit fabrication of an aspherical optic from knowledge of static behavior (3).
An updated low pressure system with a 13 mm capacitively coupled discharge was built to shape and polish single crystal silicon and silicon over silicon carbide (5). While the results for sub-surface damage were similar to previous studies (2-4) the authors also considered the evolution of surface roughness and its relationship to previous process steps. These authors found that greater sub-surface damage results in an increase in roughness.
A major limitation of the capacitively coupled discharge is the requirement that the workpiece be either conductive or less than 10 mm thick. In addition, etch rates were noted to be dependant on part thickness decreasing by a factor of ten when thickness changed from 2 to 10 mm. Above 10 mm the rates were too low to be of much use (20 nm/min) (4). Optics up to 188 mm in diameter with 30° of slope were polished under mild vacuum. With metrology used in an iterative procedure, the chamber is vented and pumped down for the next etch step. The convergence rate for PACE is typically low, resulting in a long and arduous (expensive) process. In-situ real time metrology is difficult and can require a dedicated or custom built system. While extension to larger workpieces is certainly possible, the difficulty in handling large parts is clear.
Ion beam sputtering or neutral ion beam milling removes material from the surface of the workpiece by kinetic interaction of ions with atoms or molecules of the surface. The technique has been around for quite a while (6) and the main application has been optical polishing for fused silica optics (7, 8). Earliest sources used beams with energies that were a large fraction of an MeV, while most recent systems use Kaufman sources with energies of 1500 eV to providing the optimum sputter yield. Researchers claim several advantages for ion milling. Chief among them are: no surface contact, no weight on the optic, no edge effects, high removal rate, and efficient correction of long spatial wavelength errors. The removal rates for a 1 amp beam on fused silica are about 0.35 cm3h−1; however, such high currents are rarely used. A more typical value is 30 mA providing a removal rate nearer to 0.01 cm3h−1. Zerodur was a factor of two slower. Ten hours was required for correcting (not figuring) a 30 cm optic.
As with any sub-aperture tool, the footprint must be stable and predicable. The Kaufman source, which typically produces Gaussian beams 3 to 15 cm in diameter, meets these needs quite well. The size of the beam can usually be reduced with an aperture. Configuring algorithms for the production of aspheric or non-rotationally symmetric workpieces have been extensively developed (11).
Observed disadvantages include high surface temperatures, an increase in surface roughness, and the need for a vacuum (and to do translation in that vacuum). The temperature is dependant on beam current so an increase in etch rate assumes an increase in temperature, often surpassing several hundred ° C. An increase in roughness on undamaged material occurs primarily from redeposition. This problem is largely uncorrectable because the sputtered material does not remain in the vapor phase and will, on cooling, condense on the nearest surface. Although, later efforts suggest that any amount of subsurface damage in the workpiece will degrade surface quality (9). Typically a 50% roughness increase for shallow etched parts can be expected (10). The difficulty in working under vacuum is only a practical consideration, especially for large optics. This was not a concern of the engineers at Kodak who have built an ion beam figuring chamber capable of handling a workpiece that is 2.5×2.5×0.6 m in size (8).
One investigator used a direct current plasma (DCP) at atmospheric pressure to build a device capable of thinning wafers (12). The system, called a “Plasma Jet”, uses argon as the plasma gas with a trace amount of fluorine or chlorine for reactive atom production. The main intent of the device is to do backside thinning of processed silicon wafers for smart card and other consumer applications. The industry requirement of a 200 mm wafer with a thickness less than 50 um cannot be met with any current polishing process. The defects and microcracks introduced by abrasive systems create a damage layer that is a large fraction of the desired 50 um thickness. Thin wafers produced by polishing are prone to fracture even with delicate handling. In the “Plasma Jet”, wafers are thinned in a batch mode by placing them on a platten and using planetary type motion to move the sub-aperture plasma in a pseudo random fashion across the surface. The discharge is about 1″ in diameter and the removal rate is 0 to 20 mm/min for a 200 mm wafer with a uniformity of <5%. Total material removal comes to about 30 cm3/hr.
A number of analytical methods were used to assess surface quality. Scanning electron microscopy did not show defects or scratches on either side of the wafer. X-ray photoelectron spectroscopy, used to measure surface contamination, showed no evidence of elements other than silicon. The chemistry of the plasma is quite specific. As a result, metal or carbon contamination (fingerprints) present before etching were not removed. Transmission electron microscopy did not reveal any sub-surface defects in the silicon crystal supporting the supposition that the plasma is nearly 100% chemical in nature. A number of other studies including optical microscopy and adhesion tests did not reveal any additional defects.
In its current configuration, the DC plasma is not well suited for aspheric generation or material deposition. The trace reactants are introduced along with the bulk gas and, as a consequence, are widely distributed across the discharge, substantially increasing the footprint and the minimum feature size. Electrodes that are used to establish the arc are eroded by the reactants and add particulates to the atom stream; not a problem when material removal is the primary concern and surface roughness is not an issue. Electrode erosion also causes fluctuations in plasma conditions and accounts for the reduced uniformity compared to RIE systems. Detrimental electrode reactions also preclude the use of oxygen and many other plasma gases. Finally, the discharge is not as hot as an ICP and, as a result, the production of reactant atoms is reduced.
A radio frequency (RF) plasma has been used to slice silicon and as a sub-aperture tool to polish optics (13-15). The plasma is generated around a wire or blade electrode immersed in a noble gas atmosphere that contains a trace of reactive components. The plasma converts the reactive precursors to radical atoms that react chemically with the workpiece, removing material one atom at a time. Referred to as chemical vapor machining (CVM), the electrode is brought to within 200 microns of the workpiece wherever material is to be eroded. Analysis has shown the resulting surfaces to be damage free and the process is considered to be entirely chemical in nature. A comparison of damage in silicon for polishing, sputtering, chemical vapor machining and chemical etching is reported in the literature (13). Both mechanical polishing and argon sputtering induced significant damage into the silicon surface. The damage for CVM and wet chemical etching were similar and were close to the intrinsic damage typically found in silicon used in the semiconductor industry.
The non-rotationally symmetric nature of the footprint, makes the process difficult to model and control. In the case of a blade, the footprint takes the shape of a high aspect ratio rectangle with rounded corners. Process rates are limited by the ability of the plasma to decompose the reactive precursor into radical atoms. While no vacuum is required, the workpiece must be enclosed in a vessel to contain the plasma atmosphere.
The inductively couple plasma (ICP) discharge in its most familiar commercial form was originally developed to grow crystals (16, 17). In a configuration remarkably similar to the excitation source used in current analytical spectrometers, a powder of the crystal to be synthesized was aspirated into the center of the discharge. Reed (16, 17) was able to grow boules from 5 to 15 mm in diameter and as long as 30 to 90 mm with a growth rate of 20 to 50 mm/hour. No mention was made of crystal quality. This approach to crystal growth seemed dormant until the early part of this decade when it was used to produce crystalline films of a number of oxides (MgO, ZrO2, NiO, SnO2, TiO2, ZnCr2O4, Cr2O3, CoCr2O4, NiCr2O4, and several rare earth oxides) (18, 19). X-ray diffraction was used to confirm the crystalline nature of the films. Superconducting thin films of Bi—Pb—Sr—Ca—Cu—O were also fabricated with plasma spray methods (20). Between the two efforts, an ICP was used on several occasions to produce ultra fine particle by desolvating droplets aspirated into the discharge (21).
The conditions of the ICP make it ideal as a source for reactive atoms needed for shaping of damage free surfaces. In his initial work Reed surmised that the high electrical conductivity of partially ionized gases (for argon this value is 120 ohm/cm−1 at 15,000° K.) contributed to the ease of plasma formation at high pressures. There are no electrodes and a number of gases can be used as the host matrix; although argon is usually the principle component. A typical discharge is characterized by high current (100 to 1000 amps) and a relatively low voltage (10 to 100 volts). The flowing plasma is not in complete thermodynamic equilibrium; however, ion and excited state atom populations are within 10% of equilibrium values. Electron densities are high (above 1015 cm−3 is typical) suggesting equilibrium temperatures above 9000° K. (22, 23). Reed calculated a peak temperature of 10,000° K. from the ration of emission intensities for a set of argon lines, again assuming equilibrium, making the ICP an efficient source for the generation reactive atoms.
A number of characteristics can be identified as important to the process and are summarized for the various techniques in table 1. Typically, all of the approaches have one or more areas where they excel. In the case of low cost consumer optics where cost is always an issue, traditional methods such as lapping and polishing will be used for many years. For many applications, such as high fluence optics, the categories of sub-surface damage, contamination and figure control would be emphasized. Unfortunately, none of the current techniques is adequate.
TABLE 1Comparison of finishing and shaping methodsSub-MaterialFigureRemovalsurfaceSurfaceContami-VacuumSetControlRatesDamageRoughnessnationFootprintRequiredDiamondModerateVeryVeryHighGoodCutting1 to 100NoMachiningGoodHighFluidsumLappingLargeVeryModerateVeryModerateModerate25 mm toNoandGoodto HighHighFullGrindingApertureChemicalLargeNoneHighNoneGoodLowFullNoEtchApertureTraditionalSmallFlat orLowHighExcellentHigh25 mm toNoPolishingSphericalFullOnlyApertureRIEModerateFlat OnlyModerateLowGoodLowFullYesto LowAperturePACESmallGoodLowLowGoodLow25 mm toYesFullApertureIon MillingLargeGoodLowModerateGoodLow to100 nmYesto HighModerateto 50 mmPlasma JetSmallFlat OnlyHighNoneModeratePossible10 mm toNo25 mmPlasmaModerateModerateModerateNoneGoodPossibleLong andEnclosedCVMto Smallto LowNarrowAtm